Absorption of shock caused by uneven road surfaces, as well as so-called off road traction, has a great influence on the performance of a tire used for an off road motorcycle. For example, in order to keep the motorcycle going straight on an uneven road surface, tires having the following combination of specifications are used for an off road motorcycle of 250 cc displacement.    Size: 21 inches for a front wheel/19 to 18 inches for a rear wheel (a difference of greater than or equal to 2 inches is provided between the front wheel and the rear wheel)    Construction: a tire having a bias carcass for both the front and rear wheels    Aspect ratio: 100% for the front wheel/90 to 100% for the rear wheel
In order to further improve off road traction, it is effective to decrease the aspect ratio of the rear wheel tire so that the traction loss at the side portions of the tire is reduced. However, due to the construction of the tire, the rigidity of the tread portion is increased as the aspect ratio of the tire is decreased, thereby leading to considerable deterioration in so-called off road cushion characteristics (shock absorption). In addition, cushion characteristics are further deteriorated by this decrease in the aspect ratio (due to a reduction in thickness of the tire side portions), and therefore, it has been considered difficult to set the aspect ratio below the current aspect ratio, i.e., 100 to 90%.